Variable beam electron gun



Aug. 15, 1961 A. L. EICHENBAUM VARIABLE BEAM ELECTRON GUN 3 Sheets-Sheet1 Filed NOV. 20, 1958 w! U 4 x kw F U 7 N W M f: w p ,m M M 8 3 Wm M w:4/ 0 6 D 0 2 0w w .5, F- Ma L E 5 ML IL 5! V a! ,M J 3 a, E My. 3 0 3 mmc #5 n 3 4 y 2 m r) INVENTOR. HRIE L. E1 EHENBHUM an MAM/v4 54:07:00:

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n ma N WE m E L Aug. 15, 1961 A. L. EICHENBAUM VARIABLE BEAM ELECTRONGUN 3 Sheets-Sheet 3 Filed Nov. 20, 1958 o to INVENTOR. BRIE L. E1EHENBHUM jd M United States Patent Qfiice 2,996,640 Patented Aug. 15,1961 VARIABLE BEAM EIJEC'IRON GUN Arie L. Eichenbaum, Princeton, N.J.,assignor to Radio Corporation of America, a corporation of DelawareFiled Nov. 20, 1958, Ser. No. 775,195 9 Claims. (Cl. 315-45) Thisinvention relates to electron guns, and particularly to a novel andimproved means for controlling the trajectory and shape of high densityelectron beams.

High density electron beam-type amplifiers and oscillators generallyrequire electron guns which provide electron beams having uniformdensity, laminar rectilinear flow, and a prescribed divergence orconvergence angle. As presently known, most electron beam-type microwaveamplifiers, such as the traveling wave tube and the klystron, employ aPierce type of electron gun. The Pierce type electron gun uses shapedelectrodes having a specified geometry to control the flow and focusingof the electrons in the tube thus producing a uniform rectilinearelectron flow. According to the Pierce principle, an ideal electrodearrangement provides a zero equipotentia-l surface disposed atapproximately a 67 /2 angle with respect to the normal to the cathodeemitting surface at the periphery thereof. (Refer to Vacuum Tubes by K.R. Spangenberg, 1948, pages 450-458.) The predetermination of theelectrode arrangement in the electron gun generally fixes the shape ofthe electron beam so that the beam is initially convergent or divergentat a fixed angle.

It is desirable to have an electron gun which allows simple controlexternal to the electron tube for shaping an electron beam. An externaladjustable control to vary the initial divergence or convergence of anelectron beam may also be advantageous in reducing the noise figure ofan electron tube.

An object of this invention is to provide a novel and improved electrodeconfiguration for an electron gun.

Another object of this invention is to provide an improved electron gunhaving a control external to the electron tube which can vary theinitial divergence or con vergence of the electron beam.

Another object is to effect a reduction in the noise figure of anelectron tube.

According to the invention, a bearnfocusing electrode is positionedbetween the conventional beam-forming and accelerating electrodes of aPierce-type electron gun. The configuration of the electrodes is suchthat when the voltage of the beam-focusing electrode is varied, theinitial angle of divergence or convergence of the electron beam iscontrollably changed, Means external to the electron tube are providedfor adjusting the voltage of the beam focusing electrode thus affordinga convenient control for modifying the shape of the electron beam.

The invention will be described in greater detail with reference to thedrawing wherein:

FIG. 1 is a transverse sectional view of an electron gun according tothe'invention, cut in half, showing the configuration of electrodesrequired for an electron beam which may be varied from 10 convergent to20 divergent;

FIG. 2 is a transverse sectional view of an electron gun, cut in half,showing the same configuration of electrodes as in FIG. 1, includingzero equipotential surfaces for several convergence-divergence angles asmeasured in an electrolytic tank;

FIGS. 3 and 4 are graphs including curves indicating the voltagesapplied to the beam-forming and beam-focusing electrodes of the electrongun to produce a particular convergence-divergence angle of the electronbeam;

FIG. 5 is a. transverse sectional view of an electron gun, cut in half,showing the configuration of electrodes required for an electron beamwhich may be varied from convergent to 30 convergent; and

FIGS. 6 and 7 are partial sectional views of electron tubes positionedin a magnetic field and incorporating electron guns similar to thoseillustrated in FIGS. 1 and 5, respectively.

Similar elements are indicated by similar reference charactersthroughout the drawing.

An embodiment of this invention is shown in FIG. 1, in half section,wherein an electron gun comprises a cathode electrode 11, anaccelerating electrode 13, a beam-forming electrode 15, and abeam-focusing electrode 17. The various elements of the electron gunstructure are supported as a unit within a tube envelope. The cathode 11consists of a narrow cylindrical portion 19 joined with a broadercylindrical portion 21, the narrow portion 19 being solid whereas thebroad portion 21 is hollow. The solid cylindrical cathode portion 19flares outwardly at one end to form a frustoconical portion 23 having asa base a transverse surface which is coated with a thermionic emittingmaterial to provide a cathode emitting face 25. The emitting face 25lies in a plane substantially perpendicular to the longitudinal axis ofthe cathode 11, hereinafter designated the Z axis. A heater filament(not shown) is positioned within the hollow cylindrical section 21 forsupplying heat to the cathode 11 to cause emission of electrons from thecathode emitting face 25. During operation of the tube, a high densityelectron beam which may be initially convergent or divergent isprojected along the Z axis from the emitting face 25.

The accelerating electrode 13 is an annular electrode in the form of adisk having a centrally located aperture 29, which is larger in areathan the emitting face 25, to allow passage of the electrons emittedfrom the cathode face 25. The accelerating electrode 13 is spaced fromthe emitting face 25, preferably at a distance greater than the diameteror width of the face 25, along the Z axis in the direction of electronbeam, and is mounted concentric to the Z axis, The position and thevoltage of the accelerating electrode 13 are factors in determining theshape of the electron beam and the total current drawn.

Fixed closely adjacent to the narrow portion 19 of the cathode 11 is thebeam-forming electrode 15 which is generally conical, in the exampleshown in FIG. 1, and has a relatively small central aperture 31. Theelectrode 15 is so positioned that its aperture 31 is spaced behind thecathode face 25, as shown in FIG. 1. The beamforming electrode 15 isshaped in such a manner as to produce a required zero potential surfaceadjacent to the cathode emitting face 25 According to this invention, abeam focusing electrode 17 having a relatively large aperture 33 ismounted coaxially with and intermediate the accelerating electrode 13and the beam-forming electrode 15. The beam-focusing electrode 17 ispreferably an annular disk which is positioned substantially in theplane of the cathode emitting face 25. Small changes of voltage appliedto the electrode 17 shifts the zero equipotential surface substantiallythus varying the electron beam shape.

A source of direct current voltage 34 provides suitable potentials toeach of the electrodes. In the preferred embodiment of the invention,the cathode 11 and the accelerating electrode are maintained at constantvoltages, whereas the voltage applied to the beam-forming electrode 15and beam-focusing electrode 17 may be varied.

In FIG. 2 there are shown geometrical relationships of various zeropotential surfaces for corresponding convergence-divergence angles ofthe electron beam that were determined by measurements made in anelectrolytic tank, which method is well known in the art. Thus, for aparallel rectilinear beam having a zero convergencedivergence angle, azero equipotential surface 35 forms a 67 /2 angle relative to the Z axiswithin a beam radius from the periphery of the cathode emitting surface25. With the electrode configuration illustrated, measurements indicatethat at distances of about 3 beam radii from the periphery of thecathode face 25, the Zero equipotential surface 35 for zeroconvergence-divergence becomes arcuate and defines a larger angle than67 /z with respect to the Z axis.

The geometrical arrangement of the electrodes shown in FIGS. 1 and 2creates zero equipotential surfaces corresponding to electron beamangles of convergence-divergence between 20 divergence and convergencedepending upon the potentials of the beam-forming elec trode and thebeam-focusing electrode 17. As illustraed in FIG. 2, the Zeroequipotential surface for a divergent beam is represented by the linedenoted as 20. The beam-focusing electrode 17, in this embodiment ofelectron gun shown by way of example, is located close to the Zeropotential surface for an electron beam having a 10 divergent flowdenoted as 10. It is understood that the beam-focusing electrode 17 maybe positioned along any other equipotential surface provided that it islocated in the approximate center of the desired convergence-divergencerange, for example, from 20 divergent to 10 convergent as show in FIG.2. The zero equipotential surface for a 10 convergent beam is denoted as+10 in FIG. 2.

The graph of FIG. 3 illustrates the voltages which are applied to thebeam-forming electrode 15 and beamfocusing electrode 17 of theconfiguration of FIGS. 1 and 2 to produce a beam within the range of 10convergence to 20 divergence, when the cathode is maintained at zeropotential. In the example shown in FIG. 1, with a voltage on theaccelerating electrode 13 of one volt positive with respect to thecathode, the beam-focusing electrode 17 is set at about .10 voltnegative relative to ground, and the beam-forming electrode is set atapproximately .25 volt negative relative to ground to produce a beamhaving a 10 divergence angle. The ordinate values of the graph of FIG. 3represent normalized potentials VBF VACC

where for a given angle of convergence-divergence, V is the voltageapplied to the beam-forming electrode 15 as indicated by a curve a, V isalso the voltage applied to the beam-focusing electrode 17 as read oncurve b, and V is the voltage of the accelerating electrode 13. Thus,any change in the accelerating electrode voltage requires a proportionalchange in the beam electrodes 15 and 17 for the same electron beam flowangle.

It is noted that the voltage applied to the beam-forming electrode 15 isvaried to a smaller degree in relation to the changes in potentialapplied to the beam-focusing electrode 17 While holding the beam-formingelectrode 15 at a constant potential of about .30 volt negative relativeto the cathode, measurements made with the electrolytic tank indicatethat variations in potential from about .53 volt negative to about .22volt positive applied to the beam-focusing electrode 17 change the angleof the electron beam from 10 convergent to 20 divergent, as shown bycurves c and d in FIG. 4.

Other combinations of electrodes and applied potentials may be used fordifferent desired ranges of electron beam convergence-divergence. As anexample, a configuration of electrodes such as disclosed in FIG. 5 maybe employed, wherein the cathode emitting surface 37 is concave and aidsin converging the electron beam, to produce a high density electron beamin the range of 10 to 30 convergence. The beam-focusing electrode 17 andthe beam-forming electrode 15 are set at smaller angles relative to theZ axis than in the arrangement of electrodes for the case of 10convergence to20 divergence. However, in every application of theinvention, the zero potential surface for a particular electron beamangle of convergence-divergence is located between the beam-formingelectrode 15 and the beam-focusing electrode 17.

To provide an electron gun having a maximum divergence angle greaterthan 20, the beam-forming electrode 15 is located further away from thebeam-focusing electrode 17. If a greater convergence angle than 10 isrequired, the beam-focusing electrode 17 may be located along the 0 or 5convergence zero potential surface. A total range of 40 divergence to 10convergence may then be covered with an electron gun, according to theinvention, by the simple adjustment of the potentials of thebeam-forming electrode 15 and the beam-focusing electrode 17. By the useof a gun structure that produces a beam that is divergent in the regionnear the cathode 11, the noise figure of the tube is substantiallyreduced.

In microwave beam tubes, the beam from a magnetically shielded Piercetype gun normally enters a strong axial magnetic field near a pointwhere the radius of the beam is a minimum, so that magnetic focusingforces largely determine the beams subsequent behavior. The initialformation of the electron beam is determined principally by the electricfields which are established according to the shapes, potentials, andarrangement of the gun electrodes.

When the electrons of a beam which enter the magnetic focusing fieldhave a transverse velocity component, the Brillouin flow is affected anda poorly formed, scalloped beam may result. The Brillouin flow isdefined as a magnetically focused electron stream in which all electronsare considered to be rotating in circles concentric with the axis withuniform angular velocity and have the same axial component of velocity.Forces acting on electrons are in balance at all radii. However, arandom transverse electron velocity distribution in the beam results ina flow condition differing appreciably from this ideal picture, and thescalloping effect occurs.

The electrode configurations described herein afford the advantage ofproviding proper Brillouin flow when utilized in an electron tube whichis positioned in a magnetic field. In each of FIGS. 6 and 7, a magneticfield is shown provided by an external magnetic coil 39 surrounding atube envelope 40. The gun comprises an accelerating electrode 4 1 in theform of a cylindrical cup of magnetic material having a central aperture43. The cup 41 encloses the cathode 11, the beam-forming electrode 15and beam-focusing electrode 17 to serve as a magnetic shield therefor.The fringing magnetic field B produced by the coil 39 in the aperture 43deflects the electrons which pass through the aperture 43 of theaccelerating electrode 4-1 into spiral paths to produce a constantdiameter beam with Brillouin flow.

A feature of this embodiment incorporating the invention is that thelocation of the magnetic shield or accelerating electrode 41 is notcritical because the plane of minimum beam diameter may be shifted byvarying the potentials of the beam-forming electrode 15 and thebeam-focusing electrode '17. Also, by employing the desired gun designand potentials, proper boundary conditions along the edge of the beam atthe cathode are established resulting in uniform emission and awell-collimated beam. Since the electrons of the beam arrive at theaperture of the accelerating electrode with a small radial velocity as aresult of" the adjusted flow angle, a laminar flow of electrons with auniform current density over the cross section of the electron beam isprovided upon entry into the magnetic field B causing a consequentreduction in scalloping. Also, since there is a laminar rectilinear flowof electrons towards the target, a magnetic field of lower intensity isrequired for focusing. Therefore, a smaller magnet of less weight andrequiring less power may be employed to provide a weaker magnetic field.

It is understood that the invention is not limited to the geometries ofthe electrodes shown by way of example, but is applicable to variouscombinations of electrodes employed in an electron gun which includes abeam-forming electrode with an aperture located behind a cathodeemitting face and a beam-focusing electrode having an aperturesubstantially in the plane of the cathode emitting surface with meansexternal to the tube for varying the potential of the beam-focusingelectrode.

What is claimed is:

1. An electron gun for producing and shaping an electron beam comprisinga cathode having an electron emitting surface with a predeterminedwidth, a beam-forming electrode and a beam-focusing electrode, saidelectrodes each having a single aperture in a plane located adjacent tosaid emitting surface at a distance from said surface less than one halfof said width, and terminal means for applying different variablevoltages to said electrodes to vary the convergence-divergence angle ofthe beam.

2. An electron gun contained within an electron tube for projecting avariable-shape electron beam along a predetermined axis comprising acathode having an emissive surface, a plurality of spaced aperturedelectrodes including a beam-forming electrode, a beam-focusing electrodeand an accelerating electrode coaxially mounted relative to said axisand spaced from said emissive surface, the aperture of saidbeam-focusing electrode being substantially in the plane of saidemissive surface and positioned between the other two of said aperturedelectrodes, and terminal means external to said tube for applyingdifferent potentials to said electrodes, and means for varying thepotential applied to said beam-forming and beamfocusing electrodes tovary the convergence-divergence angle of said electron beam.

3. An electron gun for projecting an electron beam comprising a cathodeelectrode having an emitting face, a beam-forming electrode having anaperture therein, a beam-focusing electrode having an aperture therein,the planes of said apertures being spaced from said cathode emittingface at distances less than one half of the width of said emitting face,an accelerating electrode positioned in front of and spaced from saidcathode face at a distance greater than said width, means for applyingdifierent potentials to said electrodes, and means for separatelyvarying the potentials of the beam-forming and beam-focusing electrodesto vary the convergence-divergence angle of the electron beam.

4. An electron gun in an electron tube for producing a high densityelectron beam comprising a cathode elec trode having an emitting face,an accelerating electrode spaced from and in front of said emittingface, a beamforming electrode disposed behind said emitting face, abeam-focusing electrode positioned between said accelerating andbeam-forming electrodes, means for applying potentials to saidelectrodes, and means external to said tube for varying the potentialapplied to said beam-focusing electrode whereby the angle of convergenceor divergence of said electron beam is changed.

5. An electron gun for providing an electron beam comprising a cathodeelectrode having an emitting face for producing an electron beam, abeam-forming electrode having an aperture closely spaced from anddisposed behind said cathode emitting face, an accelerating electrodedisk positioned in front of said cathode emitting face having anaperture larger in diameter than said beamforming electrode aperture, abeam-focusing electrode positioned between said beam-forming andaccelerating electrodes and having an aperture substantially in theplane of said emitting face, means for applying voltages to saidelectrodes, and means for varying the voltage applied to saidbeam-focusing electrode whereby the convergence-divergence angle of saidelectron beam is varied.

6. An electron gun for projecting a variable-shape electron beam along apredetermined axis comprising a cathode having an emissive surfacetransverse to said axis, an apertured accelerating electrode spaced fromsaid cathode, an annular beam-forming electrode having an aperture, anannular beam-focusing electrode between said accelerating electrode andsaid beam-forming electrode and lying substantially in the transverseplane of said emissive surface, whereby said gun can be controlled toprovide either convergent, divergent or parallel flow in the regionbetween said cathode and said accelerating electrode by the applicationof suitable voltages to said beam-forming and said beam-focusingelectrodes.

7. An electron gun for producing an electron beam comprising a cathodeelectrode having an emissive surface of approximately .025 inchdiameter, the longitudinal axis of said cathode electrode passingthrough the approximate center of said surface at a normal to the planeof said surface, an accelerating electrode having an aperture spaced infront of said emissive surface at a distance of about .041 inch, abeam-forming electrode having an aperture surrounding said cathode, theplane of said beam-forming electrode aperture parallel to and spacedfrom said emissive surface at about .003 inch, a beamfocusing electrodedisposed between said accelerating electrode and said beam-formingelectrode having an aperture, the plane of said beam-focusing electrodeaperture positioned substantially in the plane of said emissive surface,means for applying voltages to said electrodes, and means to vary thevoltages of the beam-forming and beam-focusing electrodes therebychanging the shape of said electron beam.

8. A variable shape electron beam producing device comprising anelectron gun having a cathode electrode with an electron emittingsurface, a plurality of electrodes having apertures spaced closely fromsaid cathode emitting surface, means for applying variable potentials tosaid plurality of electrodes, to vary the convergence-divergence angleof the beam, a cup-shaped accelerating electrode of magnetic materialmagnetically shielding said electrodes and having an aperture alignedWith said first named apertures, and means for providing a magneticfield along the axis of said device for producing a uniform diameterelectron beam having Brillouin flow in the region beyond the aperture insaid cup-shaped electrode.

9. An electron gun for projecting a variable-shape electron beam along apredetermined axis comprising a cathode having an emissive surfacetransverse to said axis, an apertured accelerating electrode spaced fromand in front of said cathode, an annular beam-forming electrode, and anannular beam-focusing electrode between said accelerating electrode andsaid beam-forming electrode, the apertures in said annular beam-formingand beamfocusing electrodes being disposed close to the plane of saidemissive surface, whereby said gun can be controlled to produce variousdegrees of convergence or divergence, or parallel flow, in the regionbetween said cathode and said accelerating electrode by the applicationof suitable potentials to said beam-forming and beam-focusingelectrodes.

References Cited in the file of this patent UNITED STATES PATENTS2,094,606 Knoll Oct. 5, 1937 2,268,197 Pierce Dec. 30, 1941 2,323,986Flory July 13, 1943 2,400,753 Haefi May 21, 1946 2,452,619 Weimer Nov.2, 1948 2,567,674 Linder Sept. 11, 1951 2,800,602 Field July 23, 19572,811,667 Brewer Oct. 29, 1957

